1. Field of the Invention
The present invention relates to a charging device for use in an image forming apparatus such as a printer, a copying machine or a facsimile machine adopting an electrophotographic system, and an image forming apparatus using the charging device. Particularly, the invention relates to a contact type charging device which comes in contact with the surface of a member to be charged so as to charge the surface of the member to be charged, and an image forming apparatus using the charging device.
2. Description of the Related Art
Of a related-art image forming apparatus of this type such as a printer, a copying machines or a facsimile machine adopting an electrophotographic system, as an apparatus that can form color images at high speed and with high image quality, various so-called full-color tandem machines have been proposed and manufactured practically. In such a tandem machine, four image forming units for forming toner images of respective colors of yellow, magenta, cyan, and black are disposed horizontally in parallel with one another. The toner images of the respective colors of yellow, magenta, cyan and black formed sequentially on the respective image forming units are once primary-transferred in multi-layers onto an intermediate transfer belt. After that, the multi-layer toner images are secondary-transferred from the intermediate transfer belt onto recording paper in a lump, and fixed onto the recording paper. Thus, a color image is formed.
However, in such a full-color tandem machine, four image forming units for forming toner images of respective colors of yellow, magenta, cyan, and black have to be disposed horizontally in parallel with one another, and an intermediate transfer belt has to be disposed under these four image forming units. It is therefore difficult to miniaturize the machine as it is, and the machine is not suitable to be produced as a desktop type printer.
Accordingly, various techniques for making it possible to miniaturize such full-color tandem machines while utilizing the feature that color images can be formed at high speed and with high image quality, that is, various techniques about so-called micro-tandem machines have been proposed, for example, as disclosed in JP-A-Hei.8-36288, JP-A-Hei.8-62920, JP-A-Hei.8-160839, and JP-A-Hei.9-325560 (corresponding to Japanese Patent Application No.Hei.8-166705).
According to such a technique about a micro-tandem machine, four image forming units of yellow, magenta, cyan, and black are disposed vertically, while recording paper is conveyed vertically by a paper conveying belt or directly. Thus, the installation area can be reduced. Further, the four image forming units themselves and photoconductor drums are reduced in diameter for realizing miniaturization so that the machine as a whole can be miniaturized.
When the diameter of each photoconductor drum is reduced thus to miniaturize the full-color tandem machine, it is necessary to reduce the diameter of each charging roll that comes in contact with the surface of the corresponding photoconductor drum so as to charge the surface of the corresponding photoconductor drum. Further, when the diameter of the photoconductor drum is reduced, a blade-like cleaning unit for removing foreign matters such as toner adhering to the surface of the photoconductor drum may be difficult to displace around the photoconductor drum. Because the photoconductor drum has a small diameter, there is also a problem that the abrasion of the drum caused by the friction with the blade becomes so conspicuous that the life of the drum is shortened. In this case, it is inevitable to adopt a cleanerless system using no blade cleaning unit or having a pseudo cleaning unit.
As the charging roll, a roll in which the outer circumference of a metal shaft is coated with a conductive rubber layer with adjusted resistivity is generally used. However, when the diameter of the charging roller is reduced, the hardness of the rubber layer is so high that the contact pressure becomes lower in the center portion than that in the opposite end portions. Thus, there is a problem that it becomes difficult to bring the charging roll into contact with the surface of the photoconductor drum uniformly in the axial direction so that charging becomes ununiform.
Therefore, in order to solve the foregoing problem and to bring the charging roll reduced in diameter into contact with the surface of the photoconductor drum uniformly, it can be considered that the outer shape of the charging roll is formed into a crown shape, or a large amount of plasticizer is added to the rubber layer so as to make the hardness lower.
In this case, however, it is necessary to work the outer shape of the charging roll into a crown shape with precision. Thus, the lowering of the yield results in increase in the cost. On the other hand, when a large amount of plasticizer is added, there arises a new problem that the plasticizer exudes to cause a trouble called xe2x80x9cbleedxe2x80x9d in which the plasticizer bleeds to have a damaging effect on charging.
Therefore, techniques for solving the foregoing problems while making uniform charging possible even if the diameter of a charging roller is reduced have been proposed, for example, as disclosed in JP-A-Hei.6-175465, JP-A-Hei.8-44142, JP-A-Hei.10-186800, and JP-A-Hei.11-125956.
The contact type charging device according to JP-A-Hei.6-175465 is configured as follows. That is, a metal core supported rotatably is disposed in a flexible tube having at least a conductive layer and a resistive layer outside the conductive layer. The tube is supported from the inside by the conductive flexible layer provided around the metal core. A power supply for applying a charging voltage to the tube to thereby charge a member to be charged in contact with the tube is connected to the metal core. The conductive layer is made shorter than the resistive layer in the same direction as the longitudinal direction of the tube.
On the other hand, the charging device according to JP-A-Hei.8-44142 is a charging device for use in an electrophotographic system as follows. That is, the charging device is constituted by a charging roller having a flexible conductive flexible sheet on the surface. The conductive flexible sheet is a laminate sheet of a low electric resistive layer as an inner layer and a high electric resistive layer as an outer layer. The surface resistivity of the high electric resistive layer is not lower than 106 xcexa9/xe2x96xa1 but lower than 107 xcexa9/xe2x96xa1, while the volume resistivity of the low electric resistive layer is not higher than 105 xcexa9xc2x7cm.
Further, the conductive roller according to JP-A-Hei.10-186800 is a conductive roller constituted by a plurality of layers laminated on one another. The conductive roller is arranged as follows. That is, at least a first layer and a second layer are provided as the laminated layers. The second layer is disposed adjacently to the outer circumferential side of the first layer, and the resistance of the second layer is higher than that of the first layer. The first layer is designed so that the outer circumferential portion of each end surface thereof makes an acute angle with the second layer.
Furthermore, the charging member according to JP-A-Hei.11-125956 is a charging member constituted by a conductive foam material and a conductive resin material applied onto the conductive foam material and for coming in contact with a member to be charged so as to charge the member to be charged. The charging member is arranged as follows. That is, the electric resistance of the conductive foam material is not higher than 9xc3x97105 xcexa9. The conductive resin material is made conductive by a conductive material so that the surface electric resistance of the conductive resin material is not lower than 1xc3x97105 xcexa9/xe2x96xa1 and not higher than 9xc3x97107 xcexa9/xe2x96xa1. In addition, the product hardness as a conductive member is not higher than 90xc2x0 in Asker F.
However, the related art has problems as follows. That is, any charging device disclosed in JP-A-Hei.6-175465, JP-A-Hei.8-44142, JP-A-Hei.10-186800, and JP-Hei.11-125956 can be indeed applied to a general charging roll whose diameter is about 12 mm or 14 mm. But it is difficult to make the diameter of the charging roll smaller than that. Particularly in a charging roll having a very small diameter not larger than 10 mm, there is a problem that leakage is produced between a shaft located in the vicinity of power feeding electrodes, and the surface of a photoconductor drum.
To solve such a problem, it can be considered that a charging roll 1000 from one end surface to the other end surface is covered with a surface layer 1001. In this case, as shown in FIG. 18, the surface layer 1001 projects beyond the end portions of the charging roll 1000. Thus, there occurs a failure in the outer shape, uneven contact between the charging roll and a photoconductor drum surface, or a change of the outer shape due to an environmental change, such as depression of the surface layer due to the shrinkage of the air confined in the surface layer in the charging roll end portions at the time of low temperature. In addition, because the charging roll from one end surface to the other end surface is covered with the surface layer, there is a new problem that the processing cost increases.
Further, any charging roll disclosed in the respective publications is required to come in uniform contact with the surface of the photoconductor drum. However, the smaller the diameter of the charging roll is made, the more difficult it is to keep the accuracy of the outer shape of sponge or a tube forming the elastic layer or the surface layer. When the charging roll rotates in contact with the surface of the photoconductor drum, the surface of the charging roll is deformed unevenly. Thus, there arises another problem that charging unevenness corresponding to the pitch of the roll is apt to appear as shown in FIG. 19.
Therefore, the invention was developed to solve the foregoing problems belonging to the related art. It is an object of the invention to provide a charging device which can prevent leakage from its end portions with a simple structure even if the diameter of a charging roll is made smaller, and which is superior in uniformity of charging, and to provide an image forming apparatus using the charging device.
In order to solve the problems, according to a first aspect of the invention, there is provided a charging device having a roll-like charging member disposed in contact with a surface of a member to be charged and for charging the surface of the member to be charged,
wherein the charging member has:
a conductive substrate;
an elastic layer; and
a surface layer,
wherein the elastic layer and the surface layer are applied sequentially onto a surface of the conductive substrate;
wherein opposite end portions of the surface layer axially project beyond opposite end portions of the elastic layer, respectively; and
wherein the opposite end portions of the surface layer cover the elastic layer and open.
According to a second aspect of the invention, there is provided the charging device according to the first aspect, wherein a relationship among an outer diameter of the conductive substrate in a nip portion of the charging member, a thickness of the elastic layer and a thickness of the surface layer satisfies:
the outer diameter of the conductive substrate in the nip portion of the charging member greater than the thickness of the elastic layer+the thickness of the surface layer.
According to the second aspect of the invention, the relationship among the outer diameter of the conductive substrate in the nip portion of the charging member, the thickness of the elastic layer and the thickness of the surface layer satisfies:
the outer diameter of the conductive substrate in the nip portion of the charging member greater than the thickness of the elastic layer+the thickness of the surface layer. Accordingly, the charging roll can be made smaller in diameter. At the same time, with the conductive substrate made smaller in diameter, the charging roll can be made lighter in weight.
In addition, according to the second aspect of the invention, when the opposite end portions of the surface layer are made open, urethane is usually used as the material for forming the elastic layer which is a layer under the surface layer. The urethane is apt to absorb moisture, and the elastic layer made of the urethane or the like is thinned so that the volume can be reduced. Thus, deformation can be prevented, and particularly deformation in the end portions can be prevented.
According to a third aspect of the invention, there is provided the charging device according to the first aspect, wherein each of projecting portions of the end portions of the surface layer is longer than the thickness of the surface layer.
When each of the projecting portions of the end portions of the surface layer is made longer than the thickness of the surface layer, an electric gradient is given to each of the end portions. Thus, leakage from the end portions can be prevented surely.
According to a fourth aspect of the invention, there is provided the charging device according to the first aspect, wherein the charging member has a higher resistance value in its opposite ends than that in a power feeding portion.
Also in this case, when the resistance value of the charging member is made higher in the opposite ends than that in the power feeding portion, an electric gradient is given to each of the end portions. Thus, leakage from the end portions can be prevented surely.
According to a fifth aspect of the invention, there is provided the charging device according to the first aspect, wherein the conductive substrate is set to have a larger outer diameter in a nip portion thereof than that in any other portion thereof.
When the outer diameter of the nip portion is set to be larger than any other portion in the conductive substrate, leakage from the end portions of the conductive substrate is apt to cause a problem. Therefore, the configuration as in any one of the first to fourth aspects becomes effective.
According to a sixth aspect of the invention, there is provided the charging device according to the first aspect, wherein the charging member is set to have a diameter not larger than 10 mm.
When the diameter of the charging member is set to be not larger than 10 mm, leakage from the end portions of the charging member is apt to cause a problem. Therefore, the configuration as in any one of the first to fourth aspects becomes effective.
Further, according to a seventh aspect of the invention, there is provided the charging device according any one of the first to sixth aspects, wherein the surface layer is formed to be longer than the elastic layer.
Further, according to an eighth aspect of the invention, there is provided the charging device according to any one of the first to seventh aspects, wherein the charging member has a ratio of a length of a charging portion to a diameter of the conductive substrate expressed by:
(length of charging portion)/(diameter of conductive substrate)xe2x89xa740.
Further, according to a ninth aspect of the invention, there is provided the charging device according to any one of the first to eighth aspects,
wherein the surface layer is bonded to a surface of the elastic layer; and
wherein an unbonded area is partially provided in the surface layer in an axial direction of the charging member.
According to a tenth aspect of the invention, there is provided the charging device according to the ninth aspect, wherein the unbonded area is provided in opposite end portions of the surface layer in the axial direction of the charging member.
Further, according to an eleventh aspect of the invention, there is provided the charging device according any one of the first to tenth aspects, wherein a voltage applied to the charging member includes only a DC voltage.
Still further, according to a twelfth aspect of the invention, there is provided the charging device according to any one of the first to eleventh aspects, wherein the charging member has a peripheral velocity difference from that of the member to be charged.
Further, according to a thirteenth aspect of the invention, there is provided the charging device according to any one of the first to twelfth aspects, wherein the charging member does not have, on an upstream side thereof, any cleaning unit for removing foreign matters adhering to a surface of the member to be charged.
Further, according to a fourteenth aspect of the invention, there is provided the charging device according to any one of the first to thirteenth aspects, wherein the upstream side of the charging member is one of a transfer unit and a pseudo cleaning unit for temporarily retaining residual toner which failed in transfer.
Incidentally, as the pseudo cleaning unit, for example, there is used a refresher brush which scrapes foreign matters adhering to the surface of a member to be charged, so as to temporarily retain residual toner or the like which failed in transfer. This refresher brush does not permanently remove the residual toner or the like adhering to the surface of the member to be charged, but retains the residual toner or the like temporarily. For example, the refresher brush is used in the following system. That is, the residual toner which failed in transfer but is retained by adhesion to the charging member or the pseudo cleaning unit is transferred onto another member by the electric potential gradient among the charging member, the member to be charged, and so on. Finally, the transferred residual toner is removed permanently by a cleaning unit provided on the surface of the member to which the residual toner has been transferred.
Further, according to a fifteenth aspect of the invention, there is provided the charging device according to any one of the first to fourteenth aspects, wherein the surface layer is made of a thin tubular member.
Further, according to a sixteenth aspect of the invention, there is provided the charging device according to any one of the first to fifteenth aspects, wherein the elastic layer is made of urethane foam.
Further, according to a seventeenth aspect of the invention, there is provided the charging device according to any one of the first to sixteenth aspect, wherein the charging member is used for one of transfer and elimination of charge.
Further, according to an eighteenth aspect of the invention, there is provided the charging device according to the first to seventeenth aspects, wherein the charging member is set so that Asker MD1 hardness of a surface of the charging member is not higher than 70xc2x0.
Further, according to a nineteenth aspect of the invention, there is provided the charging device according to any one of the first to eighteenth aspects, wherein the charging member is set so that ionization potential of a surface of the charging member is not lower than 4.8 eV.
Still further, according to twentieth aspect of the invention, there is provided an image forming apparatus having:
an image carrying body; and
a roll-like charging member brought in contact with a surface of the image carrying body, the charging member for charging the surface of the image carrying body to form a toner image on the image carrying body,
wherein any cleaning unit for removing foreign matters adhering to a surface of a member to be charged is not provided on an upstream side of the charging member;
wherein the charging member has:
a conductive substrate;
an elastic layer; and
a surface layer,
wherein the elastic layer and the surface layer are applied sequentially onto a surface of the conductive substrate;
wherein opposite end portions of the surface layer axially project beyond opposite end portions of the elastic layer, respectively; and
wherein the opposite end portions of the surface layer cover the elastic layer and open.
Still further, according to a twenty-first aspect of the invention, there is provided an image forming apparatus having:
an image carrying body; and
a roll-like charging member brought in contact with a surface of the image carrying body, the charging member for charging the surface of the image carrying body to form a toner image on the image carrying body,
wherein one of a transfer unit and a pseudo cleaning unit for temporarily retaining residual toner which failed in transfer is disposed in an upstream side of the charging member;
wherein the charging member has:
a conductive substrate;
an elastic layer; and
a surface layer,
wherein the elastic layer and the surface layer are applied sequentially onto a surface of the conductive substrate;
wherein opposite end portions of the surface layer axially project beyond opposite end portions of the elastic layer, respectively; and
wherein the opposite end portions of the surface layer cover the elastic layer and open.
Further, according to a twenty-second aspect of the invention, there is provided the image forming apparatus according to any one of twentieth or twenty-first aspect, wherein a relationship among an outer diameter of the conductive substrate in a nip portion of the charging member, a thickness of the elastic layer and a thickness of the surface layer satisfies:
the outer diameter of the conductive substrate in the nip portion of the charging member greater than the thickness of the elastic layer+the thickness of the surface layer;
wherein at least the charging member and the image carrying body are integrated into one unit; and
wherein the unit is removably attached to a body of the image forming apparatus.